System for gas sparging into liquid

ABSTRACT

A system for sparging gas into liquid as for example oxygen into waste liquid containing organic solids, wherein the gas is injected as at least one gas stream orbited around a common vertical axis to scribe a swept area bounded by circles of minimum and maximum radii, thereby forming small gas bubbles in the sparging zone.

United States Patent McWhirter et 21.

Oct. 7, 1975 SYSTEM FOR GAS SPARGING INTO LIQUID Inventors: John R.McWhirter, Westport,

Conn; Jon G. Albertsson, Grand Island, NY,

Assignee: Union Carbide Corporation, New

York, NY.

Filed: Apr. 11, 1973 App], No; 350,795

Related U.S. Application Data Conlinualionin-par1 of Ser. No. 132,483,April 8, 1971, Patv No, 3,775,307, which is a continuation-impart ofSer. No. 32,070, April 27,

1970, abandoned.

U.S. Cl. 261/87; 210/14; 210/220 BOlF 3/04 Field of Search H 261/87;210/14, 15, 220, 210/221 156] References Cited UNITED STATES PATENTS1,124,855 1/1915 Callow et a1 261/87 2,041,184 5/1936 lsenhour 2,928,6613/1960 Maclarcn H 261/87 X 3,775,307 11/1973 McWhirler et a1. 210/14Primary ExamincrFrank W. Lutter Assistant Examincrwilliam Cuchlinski,Jr. Atlorney, Agent, or Firm-J. C LeFever [57] ABSTRACT A system forsparging gas into liquid as for example oxygen into waste liquidcontaining organic solids, wherein the gas is injected as at least onegas stream orbited around a common vertical axis to scribe a swept areabounded by circles of minimum and maximum radii, thereby forming smallgas bubbles in the sparging zone.

2 Claims, 11 Drawing Figures I U.S. Patent Oct.7,1975 Sheetlof?3,911,064

US. Patent Oct. 7,1975 Sheet 2 of 7 3,911,064

US. Patent 0m. 7,1975 Sheet 3 of7 3,911,064

US. Patent SURFACE AREA-FT IFT OF GASMCTUAL) Oct. 7,1975

Sheet of 7 FACTOR N 0 Ffi slsc U.S. Patent 0a. 7,1975 Sheet 6 of 73,911,064

VOLUMETRIC FLOW RATE FTI SECXFTZ 03$ 1 1$ n ZN HOlOVd US. Patent 0m.7,1975 Sheet 7 of 7 3,911,064

FIG. IO

SYSTEM FOR GAS SPARGING INTO LIQUID CROSS REFERENCE TO RELATEDAPPLICATIONS This application is a continuation-in-part of Ser. No.l32,483 filed Apr. 8, 197] and issued as US. Pat. No. 3,775,307, whichis itself a continuation-in-part of Ser. No. 32,070 filed Apr. 27, I970now abandoned, both in the names of Jon G. Albertsson and John R.McWhirter.

BACKGROUND OF THE INVENTION This invention relates to a method of andapparatus for sparging gas into liquid in the form of small bubbles.

Mass transfer processes between gases and liquids are commonly carriedout by producing bubbles of the gas in the liquid. Mass transfer occursat the gas-liquid interfaces generated in the two-phase system, and thegreater the interfacial area, the faster and more efficiently theprocess proceeds.

The efficiency of such aeration processes is greatly improved if the gascan be introduced into the liquid as tiny bubbles on the order of 004-0.16 inches in diameter. For example, comparing bubbles of 0.1 and 0.24inches average diameters, the gas-liquid interfacial surface areaproduced by one cubic foot of gas is about 720 sq. ft. and 300 sq. ft.,respectively. The production of small bubbles with large area increasesthe rate of mass exchange between the gas and the liquid. ln gravitysystems, it also slows the rate of bubble rise due to buoyancy and henceincreases the time available for mass transfer. Once formed in the mixedliquor, tiny bubbles show less tendency to coalesce than do largerbubbles, and therefore, the area advantage of small bubbles is evengreater in practice than indicated by the initial bubble size.

When a small volume of gas is to be introduced into a large volume ofliquid, a sparging system is often used. By providing tiny orifices inthe sparger, or fabrieating the sparger of a porous metal, very smallbubbles can be produced with a large gas-liquid interfacial area. Suchspargers are commonly located at the bottom of a tank so that thebubbles rise slowly through the liquid. ln large tanks, the sparger inthe form of a long perforated pipe can be rotated slowly in a horizontalplane and thereby release a moving cloud of bubbles throughout theentire body of liquid.

Some aeration processes involve liquids containing suspended solids(hereinafter called mixed liquors) and it is very difficult to producttiny bubbles in such liquids, particularly when the solid particles aresoft and adherent. Such solids quickly clog the pores or orifices in thesparger and make the process difficult to control and expensive tomaintain. Orifices (openings) can be operated successfully in such mixedliquors if they are relatively large in diameter, e.g., Vs inch orabove. However, it is well known that bubbles produced from submergedopenings will grow to a size much larger than the openings beforebreaking away. Thus, the smallest average-sized bubbles which can beproduced by prior art sparging systems from a practically-sized, Vs inchopening, are at least A inch in diameter and afford less than 300 sq.ft. gas-liquid interfacial area per cu. ft. of

gas.

Non-limiting examples of mass transfer processes involving suspendedsolids are fermentation processes and the activated sludge process forwaste disposal.

The present invention is especially advantageous for use in an activatedsludge process wherein gaseous oxygen is dissolved in waste liquidscontaining organic solids. The benefits of small bubbles and rapid,efficient oxygen solution are very important when the oxygensource gasis high in oxygen content or is pure oxygen. With the conventional airaeration of waste liquor, the source gas is available at the cost of itscompression only. However, an oxygen-enriched source gas has a highervalue, and must be effectively utilized in the activated sludge processfor replacement of air.

The activated sludge process can benefit from easier, faster solution ofoxygen from the source gas in one or more of the following ways:

1. More complete utilization of the oxygen available in the source gas.

2. Reduced power costs for compression of the source gas.

3. Reduced power costs for agitation of the mixed liquor.

4. Higher dissolved oxygen content of the mixed liquor.

5. Reduced retention time of mixed liquor in the aeration basins.

An object of this invention is to provide an improved method of andapparatus for sparging gas into liquid, including mixed liquor.

Another object is to provide such method and apparatus in which the gasis injected into the liquid through openings sufficiently large to avoidclogging thereof, yet form gas bubbles in the bulk liquid which are nolonger than the openings and which are uniform in size.

Still another object is to provide such method and apparatus forsparging oxygen gas into waste liquor and sludge.

Other objects and advantages on this invention will be apparent from theensuing disclosure and appended claims.

SUMMARY These objects are achieved by the present invention whichemploys relatively large non-plugging gas injection openings yet iscapable of producing bubbles of about 0.040.l9 inch in diameter and ofan average size no larger than the gas injection opening diameter.

In the method aspect of the invention, at least one gas stream isinjected into liquid at lineal velocity of at least 5 feet per secondand at volumetric flow rate of at least 0.06 actual cubic feet persecond per square foot of horizontal area in a sparging zone as gasbubbles. The gas stream is simultaneously orbited at tangential velocityof 4.5-33 feet per second and at rate such that the factor N D is atleast l5 feet per second about a common vertical axis so as to scribe aswept area when projected on the horizontal plane bounded by circles ofminimum and maximum radii each perpendicular to said vertical axis. Itshould be understood that both circles may in fact be in the horizontalplane so that such projection would not be necessary.

This method includes a relationship of the total crosssectional area ofthe gas stream to the aforementioned swept area. in particular the ratioof total crosssectional area of the gas stream to the swept area is0.006-006. As a result of the foregoing steps, a multiplicity ofdiscrete gas bubbles of 1/32-A inch are formed in the sparging zone.

Also according to the method. the liquid is flowed at lineal velocity ofat least 2 feet per second downwardly through the sparging zone for gasinjection therein. and the downwardly flowing gas bubble-containingliquid is discharged from the lower end of the sparging zone.

In one particular method of this invention, the aforementioned gasstream is subdivided into a multiplicity of discrete gas streams ofl/32/1 inch effective diameter. These gas streams are orbited as amultiplicity of circles of different radii but all falling within theaforementioned swept area.

In the apparatus aspect of the invention, a vertically orientedrotatable shaft is provided having a passageway therein joined at itsupper end to pressurized gas supply means. An axial flow impeller isfixedly attached to the rotatable shaft with outwardly extending bladesintermediate the shaft upper end and lower end and being aligned to flowliquid downwardly.

A multiplicity of gas sparging arms are also fixedly attached to therotatable shaft at its lower end. Each arm extends radially outwardlyfrom and the arms are spaced around the shaft with an interiorpassageway communicating with the rotatable shaft passageway and amultiplicity of I/32 A inch effective diameter openings spaced from eachother along the arm length with the center axis of said openings havinga vertical direction component. The diameter scribed by the arm tips is0.8-] l times the diameter scribed by the impeller blade tips. In apreferred embodiment without shrouding. the horizontal center plane ofthe sparging arms is positioned beneath the impeller horizontal cen terplane no more than a distance equal to the impeller blade tip scribeddiameter. The gas sparging arms and injection openings are arranged soas to scribe a multiplicity of circles each perpendicular to therotatable shaft but at different radii with all such circles whenprojected on the horizontal plane falling within a swept area bounded bythe circles of minimum and maximum radii. As with the method ofinvention, gas injection openings may all rotate in the same horizontalplane so that projection thereto is not necessary.

In this apparatus the gas injection openings are also provided insufficient size and number such that the ratio of total cross-sectionalarea of the openings to the aforementioned swept area is O.()O6O.()6.The apparatus also includes a multiplicity of fixed vertical radialbaffles at spaced intervals around the lower end of the rotatable shaftto inhibit the tangential movement of liquid in this vicinity which ifexcessive would otherwise reduce mixing efficiency and mass transfereffi ciency of gas thereto.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view taken incross-sectional elevation of a ground-positioned system for sparging gasinto liquid according to one embodiment of the invention.

FIG. 2 is a plan view looking downwardly on the FIG. 1 system with aportion of the cover being cut away.

FIG. 3 is a plan view of a suitable sparger arm assembly for use withthe FIG. 1-2 system.

FIG. 4 is an elevation view taken along line 4-4 of the FIG. 3 assembly.

FIG. 5 is a schematic view taken in cross-sectional elevation of afloating system for sparging gas into liquid according to anotherembodiment of the invention.

FIG. 6 is an enlarged end view taken in cross-section of a sparger armsection. also showing the various flow and direction relationships ofthis invention.

FIG. 7 is a graph illustrating the relation between bubble sizedistribution and the factor N D representative of energy developed bythe sparger in the sparging zone.

FIG. 8 is a graph illustrating the relation between gasliquidinterfacial surface area/volume ratio and the factor N D FIG. 9 is agraph illustrating the relation between the volumetric flow rate of gasintroduction per square unit of sparging zone horizontal area and thefactor N D The relationship is shown parametrically in terms of thedownward liquid velocity through the sparging zone.

FIG. 10 is a plan view looking downwardly on one arm of an alternativesparger assembly; and

FIG. 11 is an elevation view taken along line 11-11 of the FIG. 10assembly.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings andin particular FIGS. 1-6, pressurized gas is introduced through conduit lto vertically oriented rotatable shaft 2 having passageway 3 thereinjoined at its upper end to conduit 1. Axial flow impeller means as forexample propeller 4 is fixedly attached to shaft 2 intermediate thelatters upper end and lower end with outwardly extending blades. Amultiplicity of gas sparging arms 5 are fixedly attached to rotatableshaft 2 at its lower end. Each arm extends radially outwardly from theshaft and has an interior passageway 6 communicating with the rotatableshaft passageway 3. Shaft 2 is driven by an appropriate power source asfor example motor 7.

A multiplicity of l/SL-A inch effective diameter openings 8 as forexample orifices are spaced from each other and preferably at least twocenter-to-center effective diameters along the arm length. This spacingis preferred, if it is desired to maintain distinct, separate gasinjection streams of similar size to the characteristic-ally smallbubbles formed in the sparging zone. As used herein the phrase effectivediameter includes non-circular as well as circular openings, and refersto the diameter of the largest circle which can be inscribed within thethe openings. For non-circular openings the preferred center-to-centerspacing refers to the distance between the closest of such largestcircles which can be inscribed in adjacent openings. The center of axesx-x of these openings 8 has a vertical direction component so as toscribe a multiplicity of circles (as for example 0 and C- of FIG. 3)each in a plane perpendicular to the rotatable shaft 2. A verticaldirection component of the axes is necessary to maximize the horizontalshearing action of liquid on the gas bubbles emerging from openings 8into the liquid, as discussed hereinafter in connection with FIG. 6.When all such circles C and C- are projected on the horizontal plane p-pof FIG. 4, they fall within a swept area bounded by the circles ofminimum radii r,,, and maxi mum radii r It will be noted that the FIG.3-4 sparger embodiment has all such circles in the horizontal plane p-pand projection thereto is unnecessary. However, it is contemplated thatarms 5 may be positioned inclined from the horizontal with tips eitherupwardly toward or downwardly away from the axial flow impeller 4, asfor example to form a cone-shaped assembly (sec dotted lines in FIG. 4).In this event it would be necessary to project r and r to the horizontalplane p-p to form the swept area. All arms 5 are preferably in the samehorizontal plane pp of rotation in order to minimize the difference inhydrostatic head acting upon the openings 8 within the sparging zone.

Openings 8 are provided in sufficient size and number such that theratio of their total cross-sectional area to the swept area is 0006-006,preferably with 0. I 25-20 openings per square inch of swept area in theembodiment employing spaced small orifices to discharge a multiplicityof discrete small gas streams. That is, to achieve the improvements ofthis invention the openings must be provided within certain ranges interms of total area. If the total cross-sectional area is less than0.006 of the swept area, the efficient mass transfer of gas to liquid islimited by insufficient quantity of available gas with respect to thepower required to rotate the sparger, and if the ratio exceeds 0.06 therelative quantity of gas is so high that the flowing liquid is unable toeffectively sweep the bubbles from the sparging zone rapidly enough toavoid bubble congestion and coalescence into excessively large bubbles,i.e. larger than A. inch effective diameter. Moreover for a given totalgas flow through the sparger, excessive orifice area results ininadequate orifice head loss to avoid weeping into the sparger armsthrough some of the orifices with consequent partial inactivity of thesparger.

If fewer than 0.125 openings per square inch of swept area are providedin the small orifice-type embodiment, the mass transfer of gas to liquidis also limited by either insufficient quantity of gas with respect tothe power expended to rotate the sparger (at satisfactory gas flowrates) or by the A inch diameter upper limit on the opening size. Ifthere are more than 20 openings per square inch of swept area, theopenings are so closely spaced as to result in a degree of bubblecoalescense.

In a preferred embodiment of the apparatus from the standpoint ofgas-to-liquid mass transfer rate and small bubbles, the openings 8 andVs inch diameter, the ratio of total cross-sectional area of openings tothe swept area is 0008-004 with l4 openings per square inch of sweptarea.

The size and position of the sparger relative to the impeller isimportant for obtaining high rates of mass tion gradients for high masstransfer.

In order that the sparger zone occupy the full crosssectional area ofthe liquid stream the diameters scribed by the sparger arm tips and theimpeller blade 5 tips should be approximately equal. Since the liquidstream may diverge upon leaving the impeller the sparger diameter may beslightly greater than the impeller diameter, but in any event thescribed diametcr ratio, sparger arm tips/impeller blade tips, should not10 exceed about I. 1. Similarly, the sparger diameter may be slightlyless than the impeller diameter without incurrent severe detrimentaleffects due to the aforementioned bubble congestion, poor dispersion andreduced mass transfer. However the scribed diameter ratio,

sparger arm tips/impeller blade tips, should not be less than 0.8 andpreferably should not be less than 0.9.

The divergence of the liquid streams below the impeller is accompaniedby a progressive reduction in liquid velocity at greater distances fromthe impeller. This divergence is accentuated when the impeller-spargerassembly is positioned near the bottom of a basin of liquid so as toeffectively mix and suspend solids contained therein. Reduced liquidvelocities are detrimental because the liquid does not sweep the bubblesas effectively from the sparging zone and the propellersparger systemtends to flood. For this reason when shrouding is not provided aroundthe impeller-sparger assembly, the horizontal center plane of thesparger arms should be positioned beneath the horizontal center plane ofthe impeller no more than a distance equal to the impeller blade tipscribed diameter and preferably not exceeding one-half the impellerblade tip scribed diameter. As used herein the horizontal center planeof the sparging arms is located at and passes through the horizontalcenter line of the gas passageway connector between the rotatable shaftand the lowermost sparger arm. For example in FIG. 4, the horizontalcenter plane of the sparging arms is indicated by line pp. As usedherein the horizontal center plane of the impeller blades is a planemidway between the TABLE l Rota- Axis Distance table to between OpenOpen- Sparger Impeller Impeller Spar- Opencenter- Adjacent Swept Area/ings/infi' Diameter Diameter to ger ing inches Openings- Area SweptSwept tip to tip to Sparger Nu. R,,,, R,,,,, Diameters in. Area Areatip-in. tipin. Distance-in.

l 2.76 27.5 4 and 3 2350 0.0l3 1.04 56 24 2 1.5 25.0 4 and 3 I930 0.0090.72 55 52 24 3 1.4 7.0 3 I48 0.0l9 L5] l4 l4 l2 4 1.4 8.8 3 235 0.0]6[.3 5 "5.0 8.6 3 206 0.028 2.5 l8 l8 13.25 (1 'H) 8.6 3 206 0.02l l.7 l8l8 I325 7 1.2 6.6 3 I05 0.038 305 I4 l4 l2 8 3.2 6.6 3 I05 0.019 1.52 l4l4 [2 transfer. The sparger should be located substantially within thedownward flowing liquid stream from the impeller and relatively near theimpeller where liquid velocity is high. Substantially the full crosssection of the liquid stream should be used for gas mixing in order toavoid bubble congestion to aid bubble dispersion through the bulk liquidand to maximize the concentra- To minimize vortex flow of liquid in thesparging zone, a multiplicity of vertical baffles are radiallypositioned at spaced intervals around the lower end of rotatable shaft2. These baffles may for example be in the form of narrow radial baffles9a, 9b and 9c extending from and supported by the inner wall ofcontainer 10 (support means not illustrated). These baffles are locatedat spaced intervals around the shaft 2 perimeter, e.g., 90, andpreferably positioned in a spoke-like configuration. As illustrated inFIG. 1, vertical radial narrow baffles may be located either slightlybelow 9a at the same level 9!) or above the sparger arms 8 9c.Relatively long vertical baffles 12 may be radially positioned aroundthe shaft 2 perimeter, e.g., at 90intervals using support means wellunderstood by those skilled in the art, as for example, vertical posts13. These vertical radial baffles 12 may be employed in combination withor instead of narrow baffles 9 to prevent the liquid from spinning insmall transverse circles to form vortexes. If this occurs, the relativevelocity between the sparger arms and the liquid would be significantlyreduced, thereby undesirably reducing the liquids shearing action on gasbubbles emerging from the openings 8 and reducing the developedturbulence in the wake of the sparger arms. The number and position ofvertical radial baffles to eliminate spin-flow of liquid may beempirically determined for each particular configuration, but in generalthe problem is more severe in relatively small diameter sparging zones,hence more baffles are needed. The baffles required by the apparatus ofthis invention also reduce the tendency towards flooding at high gasflow rates and at a particular N D factor level (discussed hereinafterin detail) and liquid velocity the baffles increase the maximumallowable gas flow rate before flooding occurs.

The feed liquid to be sparged with gas may be introduced at anyelevation of container through a suitable conduit 14 and is floweddownwardly by impeller 4 to the sparging zone 15. Gas-containing liquidis discharged from the lower end of zone 15 and may exit through conduit16 at the lower end of container 10, or alternatively could pass througha passageway near the upper end of container wall, or over a weir. Ingeneral, the liquid inlets and outlet may be at the same or oppositeends of the storage container 10 but should be transversely spaced fromeach other for adequate liquid residence time, e.g. on the order ofminutes.

The portion of sparged gas rising to the liquid surface and disengagingtherefrom may he released to the surrounding environment, or dischargedfrom an enclosed overhead gas space for further use, or recirculated bypump 11 to gas feed conduit 1. The gas may for example be sparged intothe liquid for stripping undesired material therefrom. In this event,the disengaged gas will usuaily be discharged from the overhead gasspace and not recirculated.

FIG. 5 illustrates floating apparatus which could be used for practicingthis sparging method in a naturally occurring large liquid body, e.g. alagoon. The same numbers have been used for identifying elements corresponding to those in the FIG. l2 apparatus. Dome or cover 16 issupported by floats l7, and the required machinery is positionedthereon, e.g. gas recirculation pump 1] and conduit 14, rotatable shaft2 with propeller 4 and sparger S positioned thereon, and motor 7 fordriving shaft 2. if vertical radial baffles were provided, they might besuspended from the dome or supported from the lagoon bottom. The gassupply means and provision for delivering electric power to the spargerassembly are not shown in the interest of simplicity. The liquid flow isindicated by dashed lines 18 and preferably extends laterally beyond thedome after downward passage through the sparging zone for receiving gasbubbles. The gas bubbles l9 flow more directly up ward to the liquidbody surface due to the upward force of buoyancy. Accordingly the gasbubbles reaching the liquid surface are captured in the overhead spacebeneath an appropriately sized dome l6 and are recirculated by pump 11.

Referring now in detail to the method of the invention and in particularFIG. 6, the gas streams are injected into liquid in the sparging zone 15at lineal velocity V, of at least 5 ft. per second (fps), for examplealong the line x.r. Lineal velocities below 5 fps. permit weeping ofliquid downwardly through openings 8 into passageway 6. As previouslyindicated, the gas is introduced at volumetric flow rate of at least0.06 actual cubic feet per second per square of horizontal area andpreferably at least 0.10 ft. /sec. x. ft. in the sparging zone. Theexpression actual cubic feet" refers to the gas volume measured underthe conditions prevailing in the sparging zone, i.e. adjusting the gasvolume at standard temperature and pressure to the temperature andhydrostatic head in the zone. The expression horizontal area refers tothe total area inscribed by the rotating sparger arm tips, and exceedsthe previously defined swept area. it will be recognized thatconsiderable power is required to achieve the aforementioned liquidshearing and turbulence effects in the sparging Zone. Unless the gas isintroduced at least at this volumetric flow rate, there will beinsufficient gas available to effectively utilize this power or torealize the high gas-to-liquid mass transfer coefficients possible withthe method and apparatus of the invention.

Simultaneously with the aforedescribed lineal velocity V the gas streamsare orbited at tangential velocity V, of 4.533 feet per second (fps).The tangential velocity for any particular gas stream may be determinedby the formula: (revolutions per second) (7r) (diameter of circle), andrepresents the sparger movement in the horizontal plane pp. With theaforedescribed vertical radial baffles 9a9c and 12, it may be assumedthat the liquid movement in the horizontal plane is substantially zeroand constitutes drag, so that the tangential velocity V, isrepresentative of the shearing action of the liquid on the gas bubblesand also representative of the turbulence produced in the liquid by therotating sparger arm. If the tangential velocity is less than 4.5 fps,insufficient shearing action and turbulence is provided to release thegas bubbles from opening 8 before the bubbles grow excessively largeand/or to subdivide any large bubbles which may be formed in he spargingzone. Where V, is greater than 4.5 fps. the shearing force in effecttears each bubble from the opening 8 mouth long before its growth alonewould produce sufficient buoyant force to cause the separation. Moreoverthe turbulence produced in the wake of the rotating arms reduces anylarge bubbles to a small size which can exist in stable form in the highliquid shear regime.

The tangential velocity V, of the orbiting gas streams should be lessthan about 33 fps. to avoid the cavitation phenomenon which occurs whenthe pressure on the trailing side of a spinning rotor arm falls belowthe vapor pressure of the liquid at that point. The resultant lowpressure gas pocket or vacuole produces large bubbles which is contraryto the objectives of this invention. Moreover, bubbles already suspendedin the liquid tend to collapse into a zone of cavitation. The thresholdvelocity for cavitation cannot be theoretically predicted with closeaccuracy because the onset is dependent upon many other factors inaddition to rotor speed, e.g. rotor shape, system pressure and gaslineal velocity. Cavitation-type spargers are well known in the art andliterature sources indicate that the rotor speed (at which cavitationoccurs) varies considerably for different apparatus.

In the factor N D N is the speed of sparger arm and impeller bladerotation (in revolutions per second), and D is the diameter of thecircle which is circumscribed by the tips of the sparger arms (in feet).This factor is representative of the energy developed by the sparger inthe sparging zone in the form of turbulence and shear of the liquidacting on the gas bubbles at the orifice mouth openings and in the wakeof the rotating arms. Accordingly the factor'N D influences the size ofbubbles discharged from the sparging zone. It has been discovered thatwith gas volumetric flow rates above 0.06 actual cubic feet persecond/ft. horizontal area and to achieve high gas-to-liquid masstransfer rates, the factor N 'D must be at least 15 ft. /sec. preferablyat least 20 ft /sec.

Another requirement of this method is flowing the liquid at linealvelocity V of at least 2 fps. downwardly through the sparging zone 15and preferably 3-7 fps. This velocity of at least 2 fps. is needed tosweep the bubbles from the sparging zone immediately after they areformed and detached. Such velocity is also needed when solids arepresent to maintain a uniform suspension in the liquid. A liquidvelocity V,, not greater than 7 fps. is preferred to insure sufficientcontact time for efficient gas transfer to the liquid and to avoidunnecessary energy loss by virtue of repeated liquid acceleration anddeceleration. The liquid lineal velocity V of course exceeds theterminal velocity of gas bubbles in liquid, i.e. about 0.8 fps. for Vainch diameter bubbles and slightly higher for larger bubbles.

The liquid lineal velocity may for example be provided by an externalpump, but in the apparatus of this invention such velocity V is providedby axial flow impeller 4 as for example a propeller. When the liquidcontains suspended solids, impeller 4 not only serves to impart thedesired downward flow velocity but also serves to mix the liquor so thatthe solid is substantially uniformly distributed therein. Impeller 4 ofcourse mixes the gas in the liquid. When the impeller serves to keepsolids in suspension, the horsepower required for this function is themajor part of the total required power input to the rotatable shaft. Forexample, in an oxygen gasbiological waste water liquor sparging systemthis requirement is on the order of 0.08 horsepower per 1,000 gallonswaste liquor.

In embodiments wherein the rotating impeller 4 and sparging arms 5 arelongitudinally surrounded by container walls and the container bottomend, the general flow pattern of the liquid in the sparging zone may bedescribed as a rolling action. After flowing downwardly through zone 15for admixture with gas bubbles from the rotating sparger arm surfaces,the liquid reaches the container bottom and flows outwardly to thecorners and upwardly along the container walls. Impeller 4 thencirculates this liquid inwardly and again down over the rotating spargerarm 5 to repeat the bubble sweeping action. The velocity of this rollingaction is low as compared to the downflow liquid velocity in thesparging zone. In embodiments lacking container walls and a bottom asfor example the FIG. 5 apparatus, this rolling liquid flow still occursbut not to as great a degree. In general, the rolling liquid flow shouldnot be appreciably greater than required to maintain solids insuspension and/or to disperse the gas bubbles uniformly through theliquid. if the rolling liquid flow is excessive, then an unnecessarilylarge quantity of liquid is being pumped with excessive powerconsumption.

As previously indicated, the ratio of total crosssectional area of thegas stream or streams to the swept area between r and r,,,,, is 0006-006and preferably 0.008-004. A ratio below 0.006 does not providesufficient gas-liquid contact area for high rate mass-transfer oreffectively utilize the power consumed in treating liquid shear andturbulence. Ratios above 0.06 do not obtain sufficient head loss acrossthe orifices to avoid weeping of liquid into the sparger arm throughsome of the orifices. Weeping is particularly detrimental when solidsare present in the liquid, because circulation of liquid through thesparger arm can be continuous. As the sparger turns, centrifugal forcepromotes circulation of liquid from orifices near the center of rotationto orifices near the tip of the arm. Solids entering the sparger armwill accumulate and eventually plug the channel through the arm as wellas a substantial number of outer orifices. According to the methodembodiment employing small discrete gas streams, 0.l25-20 gas streamsper square inch of swept area are preferably injected into thedownwardly flowing liquid as bubbles, and more preferably 1-4 gasstreams per square inch of swept area. If fewer than 0.125 gas streamsare provided, insufficient gas is available for a maximized gas transferrate to the liquid in view of the A inch diameter upper limit. If morethan 20 gas streams are used, they are so closely spaced as to produce adegree of bubble coalescence in view of the 0.006 total crosssectionalarea/swept area lower limit.

In one method embodiment, oxygen gas is sparged into waste liquorcontaining organic solids, comprising the steps of injecting amultiplicity of discrete oxygen gas streams of /8 inch diameter spacedat 2.53.5 center-to-center diameters apart at a gas flow rate of atleast 0.1 actual ft./sec.-ft. of horizontal area in the sparging zoneinto waste liquor as gas bubbles in the vertical direction, andsimultaneously orbiting the gas streams at tangential velocity of atleast 5 fps. about a common vertical axis so as to scribe a multiplicityof circles each perpendicular to the vertical axis but at differentradii with all such circles in the same horizontal plane falling withina swept area bounded by the circles of minimum and maximum radii, theratio of total cross-sectional area of the gas streams to the swept areabeing 0.008-004 with 1-4 gas streams per square inch of swept area. Thewaste liquor is flowed at lineal velocity of 3-7 feet per seconddownwardly through the sparging zone for gas injection therein, and thedownwardly flowing gas bubble-containing liquid is discharged from thelower end of the sparging zone.

It will be noted from FIGS. 4 and 6 that arms 5 are not necessarilytransversely oriented horizontally along line p-p but instead may bepositioned at included angle a to p--p. The reason for this transverseinclination is to align the sparger arm outersurface (from which the gasbubbles are discharged) parallel with the resultant direction of therelative liquid flow V,. This relationship avoids pumping of the liquideither upwardly or downwardly by the rotating sparger arms, and therebyminimizes the power required to rotate the sparger. As shown in thevector diagrams, the tangential velocity V, is only one component of therelative liquid velocity V,. The other component V is produced by theliquid impeller 4 (located above the sparger arms 5) which pumps liquiddownwardly approximately parallel to the axis of shaft rotation.

The FIG. 6 vector diagram shows how the vector angle a of transverseinclination of the sparger arms may be derived from a summation ofvelocity components V, and V,,. In contrast to FIG. 4, the FIG. 6downward velocity component V,, is not vertical and may in practice beinclined due to a horizontal velocity component imparted to the liquidby the impeller. The vector angle of V, is usually on the order of 5] 5,and the angle a of sparger arm transverse inclination is also preferablyin this range.

FIG. 6 also illustrates the preferred substantially flat cross-sectionfor the sparger arms with the major axis A normal to the gas injectionopening center axis and at least twice the length of the minor axis Aparallel pump with means for controlling the discharge rate was providedbetween the tank and the top of the column whereby the downward flowrate of water through the sparging zone could be regulated andmaintained at a desired value. Accordingly. an axial flow impeller wasnot required or provided for these particular tests, In the interests ofdemonstrating capability of these devices to produce small bubbles, theplugging problem was ignored and the circulating liquid was clean wateruncontaminated with solid particles of appreciable size.

Comparative tests were also conducted with a stationary (non-rotating)sparger. The stationary sparger was provided with sixteen very small1/32 inch diameter orifices. These orifices were drilled in eighttubular arms of a configuration similar to FIGS. 3-4 having an overallinscribed circle diameter of l 1 inches. The arms were A diameter tubeswith two orifices per arm spaced about 4 inches apart. The results ofthe tests are summarized in Table ll.

TABLE ll Gas Stream Tangential Velocity Rf Gas (ins H O Average Bubblesec. lnncr Outer Flow Lineal N D' Flow Bubble Size Bubble Gas Stream'Icst Open- Sparger fflscc. Velocity l't ft/ Size Unifor- Stabi- AreaNumber No ing ing Spargcr RPM -ft. ft./sec. sec 1 sec Inches mity lityRat|o* Rttllk1** l 8.5 25.4 Rotating 554,7 5.8 X l0 (1.56 69.3 0.3291/16 l/X Good Good 0.0l 0.83 2 9.9 29.6 Rotating (147.0 7.7 X lll' (1.7597.8 0.318 l/lo H8 Good Good 01H 0.83 3 I0 I 303 Rotating 657.0 9.4 Xl0" (L'Jl I003 0.328 H1?) l/l'i Good Good 0.0l 0.83 4 0 0 Stationary 0 045.4 0 0.503 [/8 l/4 Poor Poor 0 0 5 (l 0 Stationary 0 (I 57.0 ll 0.3921/8 l/4 Poor Poor ll 0 6 ll (I Stationary 0 (I 36.2 0 0.577 1/8 l/J PoorPoor 0 0 Ratio oi total cross-sectional area of gas streams to the sncpturea "Number of gas streams per square inch of s cpl area to such gasinjection opening. This configuration is preferred to minimize powerconsumption and stress on the rotating arms. That is, the force impartedby the liquid in resultant direction V, on the sparger arm is reduced inproportion to the projected area of the arm in the path of this force.This projected area is minimized and in this preferred embodiment isrepresented by the relatively narrow leading surface of the sparger armhaving a height of A (the minor axis).

The advantages of this invention were illustrated in a series of testsemploying a sparger similar to the FIGS. 3-4 embodiment in a systemsimilar to that illustrated in FIG. 1 for introducing oxygen gas intowater. The l 1 inch diameter sparger consisted of eight flattenedtubular arms capped at the outer ends and joined at the inner ends to ahollow hub and shaft through which oxygen was introduced. Each arm wasdrilled with eight /8 inch diameter openings equi-spaced between 1.75and 5.25 inch radii, i.e. the spacings were about four center-to-centerdiameters apart. With this sparger, the ratio of total cross-sectionalarea of the openings (and the gas streams injected therefrom into theliquid) to the swept area was 0.01, with 0.83 openings (and gas streams)per square inch of swept area. The sparger was surrounded by an l l 5/6inch inside diameter shroud.

In these tests the speed of rotation of the sparger was varied and thefluid (gas and liquid) flow rates were varied. The sparger (and shroud)was mounted in the bottom of a small cylindrical column connected at topand bottom with an adjacent tank containing water. A

It is quite evident from Table ll and photographs of the bubblespopulation that the rotating sparger containing A; inch diameteropenings produced much smaller bubbles than did the stationary spargercontaining smaller 1/32 inch diameter openings. The bubbles produced bythe rotating sparger averaged between 1/16 and A; inch in diameter whilethose produced by the stationary sparger (Test 6) were /8 to A inchdiameter.

It was also observed photographically that bubbles produced by therotating sparger are very uniform in size compared to those from thestationary sparger. Uniformity is important and desirable particularlyfor the exclusion of very large bubbles, since only a few such bubblescan account for a significant fraction of the total gas volume. Thus, asmall number of oversized bubbles will materially reduce the bubblepopula tion and interfacial area per unit volume of gas in the system.

The improved stability of the small bubbles produced by this invention,even with a very dense population of bubbles, is also quite evident fromthe data. A review of Tests 1, 2, and 3 show that the small bubbles fromthe rotating sparger remained discrete and separate despite an increasein gas flow rate of about 60%. By comparison, the larger bubbles ofstationary sparger Tests 4, 5 and 6 showed a strong tendency to coalesceand this tendency was greatly aggravated by increasing the gas rate byabout 60%. It should not be concluded from the data of Table II that theuse of H32 inch diameter orifices in the rotating sparger of the presentinvention is unsatisfactory. 1f the stationary sparger provided with1/32 inch diameter orifices had been rotated at appropriate velocity,the size of the bubbles produced thereby would have been as small as orsmaller than those produced in the A; inch orifice tests. The employment of smaller orifices in the stationary tests than in the rotatingtests emphasizes the fact that the benefits of the present inventioncannot be achieved in a stationary sparger by reducing the orifice size.

In the aforedescribed tests, eight radial baffles were provided at equalspacings around the shroud circumference in a spoke-like configurationpositioned between the shroud wall and a 2-inch inner ring. Thesebaffles were 2 inches long in the longitudinal direction (parallel tothe rotating shaft) and were positioned above the sparger to inhibitrotation of the liquid within the shroud. An attempt was made to operatethe sparger with the baffles removed. However, it was found that achimney effect" occurred at the center of the column and was caused by arapidly rising channel of huge bubbles which had coalesced near thecenter of the sparger. The vertical radial baffles prevent liquidrotation and therefore increase relative tangential velocity V, at agiven sparger RPM (a higher N D factor). Huge bubbles occur when V, islow, i.e. less than 4.5 fps. (N D 21 In other tests conducted with thesame system and observed by high-speed motion picture film, the speed ofrotation of the sparger was progressively increased from zero whilemaintaining constant fluid flow rates. The films showed that a minimumtangential velocity of about 4.5 fps. was required at the innermostopening of this particular system in order to produce the desirablecloud of stable, small diameter bubbles in the liquid. At thisrotational speed, the tangential velocity of the outermost orifice wasabout 14 fps. (N D 21 At lower velocities the aforedescribed chimneyeffect was observed with the characteristically huge bubbles.

The ability of the rotating sparger to rapidly dissolve oxygen gas intothe liquid was determined directly by measuring the dissolved oxygen inthe liquid at the base of the mixing chamber immediately downstream ofthe sparger with respect to liquid flow (point 1) and at the liquid feedpoint to the chamber about feet upstream of the sparger (point 2). Theresults of typical measurements (using data points selected at enteringliquid DO levels of about 10, 15, and 30 ppm) are summarized in Table111:

Numbers correspond to tests numbers in Table 11 These data show that theinstant method may be used to transfer oxygen gas to water over a widerange of initial DO concentrations.

Dissolved oxygen data was taken which permitted a comparison of theoverall effectiveness of the test chamber with the stationary spargerand with the rotating sparger. The following Table IV shows the DO levelof water in and out of the chamber and the DO change which occurred.Data is selected for a consistent enter ing DO level of about 10 ppm.

Numbers correspond to test numbers in Tables 11 and 111.

The Table IV test show that the rotating sparger transferred at leastfive times as much oxygen to the liquid as the stationary sparger. Itshould be noted however that the Table 1I-1\ tests do not represent aquan titative comparison of the invention with the prior art because ofthe low gas lineal velocities (0.560.91 fps.) and even more importantlythe low downward liquid lineal velocity (0.33 fps.) Under thesecircumstances the gas bubbles actually rose through the column of liquidand were not swept downwardly from the lower end of the sparging zonewith the liquid. As previously discussed, the high mass transfer ratescharacteristic of this method require gas lineal velocities of at least5 fps. into the liquid, and liquid downward velocities of velocitiesofat least 2 fps.

Another series of tests were conducted with the aforedescribed rotatingsparger system, except that all gas injection openings on each of theeight radial arms were masked except outermost openings located on aradius of 5.25 inches. This was done in order to eliminate one of themany variables which had been present in previous tests of the rotatingsparger. Am important objective of these particular tests was to studythe effect of tangential gas velocity and rotating speed on bubblediameter, and by placing all openings on the same circle the tangentialvelocity of all openings was uniform. Vertical radial baffles wereemployed above and below the rotating sparger.

The apparatus was operated with air at various rotating speeds, hence atseveral tangential velocities of the air streams, with the character ofthe gas dispersion in the liquid being recorded photographically. Foreach condition (RPM or air tangential velocity), a representative groupof bubbles were measured, and the size distribution of the bubbleswithin the group was determined. The results are shown on FIG. 7 for sixdifferent values of the factor N D as previously defined. This data isspecific for /8 inch diameter openings immersed 5.8 feet below the watersurface and for a gas lineal velocity of 12.2 fps. (NTP) through eachinjection opening. This gas lineal velocity through only eight orificesprovides a gas flow rate of only about 0.011 actual cubic feet of gasper second per square foot of horizontal sparger area. The gas flow rateis below the 0.06 lower limit specified herein, but the purpose of theinstant tests was to study bubble size distribution and not to obtain ahigh degree of mass transfer efficiency. The number of bubbles producedwas more than adequate to show the size distribution in a statisticallyaccurate manner.

The use of FIG. 7 may be illustrated as follows: at a gas streamrelative tangential velocity of 8.7 fps. (N' D 8.4 I, at least 90% ofall the bubbles within a selected sample will be (1.37 inch diameter orsmaller. Similarly, at ll.5 fps. relative tangential velocity (l\l'-'D14.6) 90% of the bubbles within a selected sample will be smaller than0.18 inch diameter. The graph is plotted on coordinates wherein theabscissa is a probability scale, so that a bubble population which fitsa normal bell-shaped distribution curve will fall on a straight line. Itis seen that at low gas stream tangential velocities in creasing from I)to 101 fps. (values of N D from I) to l L2), bubble distribution is notnormal, since inordinately large-size bubbles are formed in relativelylarge numbers. However, a small additional increase in tangentialvelocity to 1 l5 fps. (N D of 14.6) results in a sudden change in thebubble population, whereby smaller bubbles are produced which fit anormal distri bution curve. It appears that the large bubbles occurringat low relative tangential velocity are produced l by continued growthat the gas injection opening in the absence of shear forces sufficientto disengage the bubbles from the sparger, (2) by coalescence ofadjacent bubbles within the liquid after disengagement occurs, and (3)by the stable assistance of the large bubbles in the relatively lowturbulence level of the liquid in the sparging zone. At the highervelocities, the shear forces are greater and bubble growth at theopening tends to be limited to a bubble diameter not larger than theopening diameter. The smaller bubbles produced at the opening and in thehigh shear-turbulence zone in the wake of the sparger arms are lesslikely to coalesce in the liquid.

FIG. 8 is based on the same data FIG. 7. The ordinate is the ratio ofgas bubble surface area to gas quantity and is therefore an indicator ofthe gas-to-liquid mass transfer effectiveness. FIG. 8 illustrates theimportance of tangential velocity and turbulence to the presentinvention in a more dramatic manner than FIG. 7. At low values of N Dbelow about I l .2, large unstable bubbles were produced and the surfacearea of gas created in each cubic foot (NTP) of gas injected into theliquid was about 200 square feet or less at higher values of N D ofabout l4.6 and above, the small stable bubbles provide a surface area ofabout 450 square feet and higher for each cubic foot of injected gas.The transition between these two conditions is quite abrupt.

As previously indicated, the FIGS. 78 data is based on a singlevolumetric rate of gas introduction into the sparging zone. Moreover,the liquid downflow rate through the sparging zone was quite low as theobjective was to suppress rather than reverse the buoyant upward rise ofthe bubbles and thereby facilitate close in spection of bubble size.Additional tests were conducted at other volumetric flow rates of gasinto the sparging zone and again it was found that for each gas rate arather abrupt transition in bubble size occurred in a specific uniquerange of rotating speeds (or N' D It was also observed that as gassparging rates increased. the rotating speed factor N D required toreach the small, stable bubble regime increased correspondingly.

In yet another series of tests, the volumetric gas flow rate. the liquiddownflow rate and the sparger rotating speed were varied. In some tests,no impeller was employed so that the liquid downflow rate wasessentially zero. An impeller was used in other tests. and bysubstituting impellers of different pitch/diameter ratio, the liquiddownflow rate was varied independently of the sparger rotating speed.These tests were conducted in a tank 8 by 8 feet cross-section and 9feet deep filled with water to a depth of 8 feet.

The sparger was provided with eight arms each ll inches from tip toshaft center and was submerged 6.2 feet (22 inches from tank bottom). Athree-blade marine-type propeller of selected pitch/diameter ratio wasinstalled on a common shaft 11 inches above the plane of the sparger(corresponding to the aforementioned vertical distance between impellerblade and sparger arm horizontal center planes). Each arm of the spargerwas provided with 26 A; inch diameter orifices located along upper andlower surfaces thereof with variable spacing ranging from /2 inchcenter-to-center distance (for outer holes) to 1 inch (for inner holes).A source of air was provided to the hollow shaft with means to controland measure the flow rate. Provision was also made to vary and measurethe rotating speed of the shaft.

The results of the tests are shown on FIG. 9 which is a graph with N D(ftF/sec?) as ordinate and with volumetric gas flow rate (actual cubicfeet per second per square foot horizontal area of the sparger zone) asabscissa. The incipient flooding boundaries of the propeller/sparger atvarious values of downward liquid velocity are shown as a series ofcurves on the graph. FIG, 9 shows the limiting value of liquid downflowV which is required for a desired pairing of values of N D and gas flowrate. Thus, if it is desired to introduce 1.6 actual cubic feet gas persecond per square foot sparged area using a value of N D of 30, then theliquid downflow rate should not be less than about 3.5 ft./sec.

An important observation during this series of tests was the strongdependence of system performance on the liquid downflow rate. As liquidvelocity was in creased above 2 feet/second the sparger's capability todisperse gas into small bubbles increased greatly even though its speedof rotation was held constant. Therefore. the volumetric gas flow rateof the sparger could be increased to much higher values withoutnecessarily increasing the rotational speed of the sparger. Thus, inaddition to the aforementioned relationship between volumetric gas flowrate and N D which permits higher gas flow rates at higher rotationalspeeds, the gas capacity can be further increased by increasing theliquid downflow velocity.

Below the value of 2 ft./sec. liquid rate is a regime in which thedownward liquid velocity approaches the terminal liquid velocity of thebubbles. The bubbles still cannot escape upwardly by their buoyancy, andthe liquid velocity becomes too low to sweep the bubbles downwardly outof the sparging zone as rapidly as they are produced. Total floodingresults as the accumulating gas bubbles in the sparging zone coalesceinto large pockets of gas which rise into the propeller and the sys ternbecomes incapable of producing and dispersing small bubbles. If liquidvelocity is still further diminished below the terminal velocity, asteady state small bubble regime again appears at low volumetric gasflow rates, but such gas rates are far below 0.06 actual cubic feet persecond per square foot horizontal area of the sparging zone and too lowfor practical usage. The region of operability according to the methodof this invention may be identified on the FIG. 9 graph. The verticaldotted line for gas flow rate g g) =0.06 ft."/sec. ft. is the minimumvalue of this parameter for achieving an efiective balance of masstransfer and power consumption. The incipient flooding curve V 2 alsorepresents a lower limit on liquid lineal velocity needed to achievehigh mass transfer rates (K and to effectively disperse the gas bubblesfrom the sparging zone. Also, the horizontal line for the factor N' U 15is the lower limit for acceptably high mass transfer rates.

It will be understood by those skilled in the art that despite thecoupling of the impeller and sparger to a common shaft, the liquidpumping rate of the impeller can be varied independently of therotational speed by changing the pitch/diameter ratio of the impeller.On preferred practice of this invention the pitch/diameter ratio of theimpeller is at least 1.5. It has been observed that by changing thepitch/diameter ratio from 1.0 to 1.5 and holding the factor N Dconstant, the gas rate can be increased over 50% and the mass transferrate over 75%.

Although the aforedescribed tests related to sparging systems employingl/ 16 to Vi inch diameter gas injection openings, the generalrelationships set forth are believed applicable to H132 inch openingsand such may be used in the practice of this invention. Openings smallerthan l/32 inch diameter are subject to clogging where the liquidcontains solid materials. It also appears that saturation of the gas (tobe injected in the liquid as a multiplicity of streams) becomes moreimportant as the opening diameter and gas stream diameter is reduced, soas to avoid evaporation and solids disposition in the openings. Openingslarger than inch do not provide the small bubbles necessary for a highgas-liquid interfacial surface area/volume ratio under conditionswherein shear action (and not turbulence) is primarily relied on to formsmall bubbles, i.e., low tangential velocity and low N D This isillustrated by Table V which is based on the approximation that atstable operation, the mean bubble diameter is about the same as the gasinjection opening diameter. It should be recognized however that severalother factors could influence the relationship between opening diameterand stability of operation, such as hydrostatic head and gas injectionopening profile.

TABLE V Opening Diameter Corresponding Mean Area/Volume Ratio InchesBubble Dia.-ft. -ft 3 the tank wall at intervals, and various othervertical radial baffles 9a-9a were provided around the propeller-spargerassembly, also at 90 intervals.

Some tests employed an 18 inch diameter, 18 inch pitch propeler whileothers used an 18 inch diameter, 27 inch pitch propeller. Severalsparger embodiments were employed, all consisting of eight radial armsin an enscribed circle, 18 inch in diameter. These arms were fabricatedof 5/16 inch OD. tubing which was flattened to about 3/16 inch widthalong the portion of the length containing the openings to provide asubstantially flat configuration with a major axis A of 0.25 inch andminor axis A,,,, of 0.06 inch. One design used straight arms, eachcontaining twelve Vs inch diameter orifices spaced /2 inch apart (fourdiameters) in the top of the arms beginning A inch from the arm tip (96total openings). The ratio of total cross-sectional area of openings andgas streams to the swept area was 0.018 with 1.46 openings and gasstreams per square inch of swept area. This sparger was tested with theopenings facing upward, facing downward, and facing at an acute angle tothe vertical. In other tests, the above sparger was modifled by drillingthe holes completely through the tube arms, thus providing 96 openingson the top and 96 on the bottom 192 total openings). In still othertests, the number of openings was again doubled by drilling additional9% inch diameter holes through the tube arms midway between the existingopenings (384 total openings). Yet another sparger containing 192openings was tested using arms curved in the horizontal plane on aradius of about 15 /2 inches.

In some tests, the full array of narrow radial baffles 9a-c wereemployed. In other tests, only the bottom narrow baffles 9a wasretained, while in still others all narrow radial baffles were removedbut in all tests the long baffles 12 were employed.

During these tests, it was determined that outstanding performance (interms of transferring gas as small bubbles to liquid through Va inchdiameter openings) can be obtained whether the openings are provided inthe top, bottom, or both sides of the sparger arms. However, in all ofthese configurations, the center axis of the opening had a verticaldirection component and the gas streams were injected in a directionhaving a vertical component. It was also determined that narrowstraightening baffles 9a-c are not required to avoid vortex formation inlarger tanks where the mass of liquid being circulated (for sweeping thegas bubbles away from the sparging zone) is small in relation to themass of the total liquid body. However, long radial baffles 12 wereemployed and the apparatus of this invention requires some type ofvertical radial baffles at spaced intervals around the lower end of therotatable shaft.

Although preferred embodiments of this invention have been described indetail, it is contemplated that modifications of the method andapparatus may be made and that some features may be employed withoutothers, all within the spirit and scope of the invention. For example,the gas injection openings in the sparging arms need not be provided ina single longitudinal row but may instead be provided in multiple. Inthis event, the requirement of center-to-center spacings at least twodiameters refers to any pair of adjacent openings whether they be in thesame or different rows.

The specific embodiments of the invention hereinbefore described allinvolve small discrete orifice-type openings in the sparger arms, fordischarging a multiplicity of small gas streams of about the samediameter at the desired 1/32 inch diameter bubbles. 1n the preferredpractice. a single gas stream is discharged through a slot-type opening8 as illustrated in FIGS. 10 and 11. This opening extends from the outerend of sparger arm 5 toward the inner end and along trailing edge 80.The opening also extends part way around the outer end as section 8!),with the remainder closed by cap 22. The latter feature is desirablewhen the liquid contains solid material, so that any debris will beflung outwardly. It will also be apparent that a single slottype openingminimizes the possibility of plugging which might otherwise occur withsmall orifice openings. Finally, slots are in general less expensive toform than small orifices.

Table VI summarizes several method and apparatus embodiments based onthe slot-type sparger opening of FIGS. 10 11. The plants are used forthe biochemical treatment of waste water using oxygen gas in severalcovered zones or stages, with the aeration gas and the partiallyoxygenated liquor from each stage flowing cocurrently to the nextsucceeding stage in the manner described in U.S. Pat. No. 3,547,815 toJ. R. McWhirter. The biological oxygen demand (BOD) of the incomingwaste water is 140 mg/l. for plant 1, 165 mg/l for plant 2, and 1700mg/l. for plant 3. The width of open flat section 8b of the spargerslots 8 are 1.1 inch for plant 1, 0.88 inch for plant 2, and 0.75 inchfor plant 3. The total widths of the flattened end sections of thesesparger arms (comprising open section 817 plus cap section 22) are 2.15inch for plant 1, 2.25 inch for plant 2, and 2.15 inch for plant 3. Itwill be noted from FIG. 11 that in the slot-type sparger the minimumradii R,,,, is measured from the slot inner edge whereas the maximumradii R is measured from the slot outer edge. in the practice of theinstant method, the gas stream or streams are preferably orbited attangential velocity of 18 26 feet per second to achieve the desiredturbulence in the sparger zone and yet avoid a tendency towards thecavitation phenomenon, thereby insuring the formation of small bubbles.The factor N D representative of the energy developed by the sparger inthe form of turbulence and shear of the liquid acting on the gasbubbles, is most preferably in the rather than shear is the principaleffect relied on to form small bubbles.

The spargers of the aforementioned plants are mounted on verticallyoriented rotatable shafts beneath axial flow impellers, also mounted onthe shafts as illustrated in FIG. 1. The impellers are the pitch bladeturbine type, all with a 32 pitch, and the dimensions are summarized inTable VI].

" W is the \cmcal pruiectiun of the turbine blade What is claimed is:

1. A method for sparging gas into liquid comprising the steps ofinjecting into liquid one gas stream at lineal velocity of at least 5feet per second and at volumetric flow rate of at least 0.06 actualcubic feet per second per square foot of horizontal area in a spargingzone as gas bubbles in a horizontal direction, and simultaneouslyorbiting said gas stream at tangential velocity of 18-26 feet per secondand at rate such that the factor N D is 30-70 feet per second about acommon vertical axis so as to scribe a swept area when projected on thehorizontal plane bounded by circles of minimum and maximum radii,wherein N is the speed of sparger arms and impeller blade rotation (inrevolutions per second), and D is the diameter of the circle which iscircumscribed by the tips of the sparger arm (in feet), eachperpendicular to said vertical axis, the ratio of total cross-sectionalarea of said gas stream to said swept area being 0.006 0.06, therebyforming a multiplicity of discrete gas bubbles of 1/32 A inch in saidsparging zone; flowing said liquid at lineal velocity of at least 2 feetper second downwardly through said range of 30 70 for the same reasons.sparging zone for gas injection therein; and discharging TABLE V] SlotSlot End Plant and Height Length Radius Gas V Gas Flow TangV, Liqd VStage No. Hlinches) L(inchcs) r,,,,,( inches) RPM (fps) Rate* (fps)**N-D (fps) Plant 1 Stage I l 6.75 53 56 65 0.26 26 68 9 Stage 2 l 4.5 4856 97.5 0.31 23.5 56 9 Stages 3,4815 1 3.0 48 45 103 0.22 l8.8 36 9Plant 2 Stage 1 0.75 4.1 44.5 56 80 0.15 21.8 48 46 Stage 2 0.75 3.942.5 56 69 0.18 20.8 44 4.6 Stage 3 0.75 3.0 42.5 56 67 0.14 20.8 44 4.6Stage 4 0.75 3.9 42.5 56 69 0.18 20.8 44 4.6

Plant T Stage 1 1.42 3.1 48 56 11.16 23.5 56 9.5 Stage 2 1.42 3.5 48 5648 0.16 23.5 56 9.5 Stage 3 L42 4.4 48 56 43 0.18 23.5 56 9.5

()aygcn gas tlov rate, IWscc [Sq ft of hnrivontal area In sparging 70m."()x gcn gas stream tangential \elucit All plants have eight arms per sarger. each formed from 2.875 inch OD stainless steel pipe and flattenedto 2.15 2.30 inch width at the end.

Another difference between the FIG. 10 11 embodiment and thosepreviously described is that the gas is not discharged into the spargingzone in a direction having a vertical component, so that liquidturbulence the downwardly flowing gas bubble-containing liquid from thelower end of said sparging zone.

2. Apparatus for sparging gas into liquid comprising: a. pressurized gassupply means;

b. a vertically oriented rotatable shaft having a passageway thereinjoined at its upper end to said gas supply means;

. an axial flow impeller fixedly attached to said rotatable shaft withoutwardly extending blades intermediate said upper end and the shaftlower end aligned to flow liquid downwardly;

. a multiplicity of gas sparging arms fixedly attached to said rotatableshaft at its lower end, each extending radially outwardly from andspaced around the shaft with the diameter scribed by the arm tips 0.8 l.l times the diameter scribed by the impeller blade tips and each armhaving an interior passagea multiplicity of vertical radial baffles atspaced intervals around the lower end of said rotatable shaft.

1. A METHOD FOR SPARGING GAS INTO LIQUID COMPRISING THE STEPS OFINJECTING INTO LIQUID ONE GAS STREAM AT LINEAL VELOCITY OF AT LEAST 5FEET PER SECOND AND AT VOLUMETRIC FLOW RATE OF AT LEAST 0.06 ACTUALCUBIC FEET PER SECOND PER SQUARE FOOT OF HORIZONTAL AREA IN A SPARGINGZONE AS GAS BUBBLES IN A HORIZONTAL DIRECTION, AND SIMULTANEOUSLYORBITING SAID GAS STREAM AT TANGENTIAL VELOCITY OF 18-26 FEET PER SECONDAND AT RATE SUCH THAT THE FACTOR N2D2 IS 30-70 FEET2 PER SECOND2 ABOUT ACOMMON VERTICAL AXIS SO AS TO SCRIBE A SWEPT AREA WHEN PROJECTED ON THEHORIZONTAL PLANE BOUNDED BY CIRCLES OF MINIMUM AND MAXIMUM RADII,WHEREIN N IS THE SPEED OF SPARGER ARMS AND IMPELLER BLADE ROTATION (INREVOLUTIONS PER SECOND), AND D IS THE DIAMETER OF THE CIRCLE WHICH ISCIRCUMSCRIBED BY THE TIPS OF THE SPARGER ARM (IN FEET), EACHPERPENDICULAR TO SAID VERTICAL AXIS, THE RATIO OF TOTAL CROSS-SECTIONALAREA OF SAID GAS STREAM TO SAID SWEPT AREA BEING 0.006-0.06, THEREBYFORMING A MULTIPLICITY OF DISCRETE GAS BUBBLES OF 1/32-1/4 INCH IN SAIDSPARGING ZONE, FLOWING SAID LIQUID AT LINEAL VELOCITY OF AT LEAST 2 FEETPER SECOND DOWNWARDLY THROUGH SAID SPARGING ZONE FOR GAS INJECTINGTHEREIN, AND DISCHARGING THE DOWNWARDLY FLOWING GAS BUBBLE-CONTAININGLIQUID FROM THE LOWER END OF SAID SPARGING ZONE.
 2. APPARATUS FORSPARGING GAS INTO LIQUID COMPRISING: A. PRESSURIZED GAS SUPPLY MEANS, B.A VERTICALLY ORIENTED ROTATABLE SHAFT HAVING A PASSAGEWAY THEREIN JOINEDAT ITS UPPER END TO SAID GAS SUPPLY MEANS, C. AN AXIAL FLOW IMPELLERFIXEDLY ATTACHED TO SAID ROTATABLE SHAFT WITH OUTWARDLY EXTENDING BLADESINTERMEDIATE SAID UPPER END AND THE SHAFT LOWER END ALINGED TO FLOWLIQUID DOWNWARDLY, D. A MULTIPLICITY OF GAS SPARGING ARMS FIXEDLYATTACHED TO SAID ROTATABLE SHAFT AT ITS LOWER END, EACH EXTENDINGRADIALLY OUTWARDLY FROM AND SPACED AROUND THE SHAFT WITH THE DIAMETERSCRIBED BY THE ARM TIPS 0.8 - 1.1 TIMES THE DIAMETER SCRIBED BY THEIMPELLER BLADE TIPS AND EACH ARM HAVING AN INTERIOR PASSAGEWAYCOMMUNICATING WITH THE ROTATABLE SHAFT PASSAGEWAY, AND A SINGLE SLOTTYPE OPENING IN PART OF EACH ARM END AND EXTENDING THEREFROM ALONG THETRAILING EDGE TOWARD THE INNER END SO AS TO SCRIBE WHEN PROJECTED ON THEHORIZONTAL PLANE A SWEPT AREA BOUNDED BY THE CIRCLES OF MINIMUM ANDMAXIMUM RADII, SAID SLOTTYPE OPENINGS BEING PROVIDED IN SUFFICIENT SIZETHAT THE RATIO OF TOTAL CROSS-SECTIONAL AREA OF SAID OPENINGS TO SAIDSWEPT AREA IS 0.006 - 0.06, AND E. A MULTIPLICITY OF VERTICAL RADIALBAFFLES AT SPACED INTERVALS AROUND THE LOWER END OF SAID ROTATABLESHAFT.